Method and system for operative reconversion of pairs of pre-existing steam turbo-units

ABSTRACT

Method and system, in which a pair of pre-existing steam turbo-units, a first one of which is formed by a first steam turbine and a first electric generator connected on a first common shaft and a second one of which is formed by a second steam turbine and by a second electric generator connected on a second common shaft, are coupled to one another in a system configuration of a cross-compound type; in which high-pressure and medium-pressure sections of the first turbine are replaced with a single high-pressure section made in such a way as to maintain the pre-existing foundations, and in which high-pressure and medium-pressure sections of the second turbine are replaced with a single medium-pressure section made in such a way as to maintain the pre-existing foundations. A single supercritical boiler supplies in cascaded fashion one and the same flow of steam to the new high-pressure and medium-pressure sections set on the first shaft and on the second shaft, respectively. A single piping downstream of the new medium-pressure section collects the flow of steam and supplies it in parallel, via a branch thereof intercepted by a regulation valve, to the pre-existing low-pressure sections of both of the shafts.

The present invention relates to a method for obtaining operativereconversion of pairs of pre-existing steam turbo-units. The inventionmoreover relates to a system for the production of energy by means of atleast one pair of steam turbines operating in the so-calledcross-compound configuration, i.e., in which each turbine is set on aseparate shaft and operates a generator of its own.

BACKGROUND OF THE INVENTION

It is known that numerous systems for the generation of energy are basedupon a pair of turbo-units operating in tandem-compound configuration,in which each turbo-unit includes an electric generator driven on thesame shaft by a steam turbine supplied by an oil-burning or coal-burningboiler of its own, with subcritical steam conditions both on superheated(SH) steam for high-pressure (HP) admission and on re-superheated (RH)steam for medium-pressure (MP) admission. Steam turbines are generallyof the two-body type (a combined HP-MP section and a low-pressure (LP)section).

The above type of systems presents a relatively low efficiency, so thatthe tendency is to convert such a system into a system of approximatelythe same power, which presents supercritical and/or ultrasupercriticalconditions at HP and MP admission (in order to increase the efficiency)and is based upon a single boiler, which supplies both of thepre-existing turbines, reconfigured into a cross-compound configuration.

For the above purpose, the known art envisages, in addition to replacingthe two boilers with a new boiler of approximately twice the horsepower,replacement of the two steam turbines or of at least the two combinedHP-MP sections with as many new sections in order to meet up to thehigher design conditions (pressure and temperature of the steam atadmission to the HP and MP sections), for which the materials and theoriginal design of the pre-existing turbines are no longer adequate.

Said solution is not, however, free from drawbacks. In addition to beingcostly, the new steam-turbine sections present, in fact, levels ofefficiency that are panalized, as compared to the new supercritical orultrasupercritical steam conditions, by the number of stages limited bythe encumbrance of the existing foundation.

With a single boiler, which supplies the two turbo-units, the system ofregulation and the running of the turbo-units themselves togetherrepresents, then, an element of greater complexity. It is, in fact,necessary to provide manifolds for the SH and RH steam, from which bothof the turbo-units are to be supplied in branched fashion, and it isproblematical to adapt the steam conditions to the requirements of thetwo turbo-units (which may be different from one another, for example,with one unit in use and the other unit in its starting up stage after astoppage). For instance, it is necessary to double, among other things,the total number of the main valves and the number of the regulation andprotection systems.

Systems in cross-compound configuration with different lay-outs are alsoknown to the art, for example from U.S. Pat. No. 4,316,362 andJPA59-60008. The configurations known from said documents are, however,suitable only for newly-devised systems, and not for reconfiguration ofpre-existing systems. Furthermore, the systems known from said documentspresent complex lay-outs.

SUMMARY OF THE INVENTION

The aim of the present invention is to provide a method for enablingconversion of a traditional thermoelectric system fed by a subcriticalsteam cycle and based upon a pair of steam turbo-units of similar sizeinto a single turbo-units group in cross-compound configuration withapproximately twice the power, that will be free from the drawbacksdescribed and that, in particular, will enable the necessary adaptationof the steam turbo-units present to be obtained with relatively reducedinvestments, in short times, in a simple way and at the same timeachieving high levels of energy efficiency.

A further aim of the invention is to provide a system for the productionof energy that will be simple and inexpensive to set up, in particularin the case of reconversion of a pre-existing system based upon atraditional subcritical steam cycle into a supercritical system, whichwill be highly reliable and relatively simple to control.

According to the invention, a method for obtaining operativereconversion of pairs of pre-existing steam turbo-units is henceprovided according to what is defined in the attached claims.

According to the invention, a system for the production of energy bymeans of at least one pair of steam turbines operating in cross-compoundconfiguration is moreover provided according to what is defined in theattached claims.

In particular, in the system according to the invention, a first steamturbine is coupled on a first shaft to a first electric generator, and asecond steam turbine is coupled on a second shaft to a second electricgenerator. The first steam turbine includes just a high-pressure sectionand a first low-pressure section, whereas the second steam turbineincludes just a medium-pressure section and a second low-pressuresection.

The high-pressure section is connected exclusively to themedium-pressure section, upstream thereof, whilst the medium-pressuresection is connected in parallel to said first and second low-pressuresections, immediately upstream thereof, by means of a piping of arelatively short length, which connects the medium-pressure section ofthe second steam turbine to the second low-pressure section, which islocated on the same shaft, and, by means of a branching of said piping,of a relatively long length, which connects said medium-pressure sectionto the first low-pressure section, which is located on the shaft of saidhigh-pressure section.

A regulation valve intercepts said branch in series, as close aspossible to said piping.

Hereinafter, by the term “hydraulically connected” is meant a connectionthat enables a fluid (indifferently, liquid or, as in the present case,gas, i.e., in form of steam) to flow between the connected elements.Likewise, as may be seen, the terms “upstream” and “downstream” arereferred to the direction of flow of said fluid.

One such system is obtained with the method of the invention, whichcomprises: a step of decommissioning and removal of the high-pressureand medium-pressure sections of both of said steam turbines; a step ofreplacement of the removed sections with a new high-pressure section onthe first shaft and a new medium-pressure section on the second shaft,said new high-pressure section and medium-pressure section being made insuch a way as to occupy at least part of the space left free, on eachshaft, by both the high-pressure section and medium-pressure sectionremoved from that shaft, leaving the pre-existing foundations unaltered;and a step of setting in hydraulic connection in cascaded fashion thenew high-pressure section on the first shaft and of the newmedium-pressure section on the second shaft with a single supercriticalboiler, in such a way that the latter will be able to supply one and thesame flow of steam, in series, to the new high-pressure section and,then, to the new medium-pressure section.

The advantages as compared to the known art are the following:

-   -   the new sections of turbine (an HP body replacing the HP-MP body        of the first shaft and an MP body replacing the HP-MP body of        the second shaft) enable optimized levels of efficiency of the        individual sections to be achieved on account of the reduction        in the secondary losses (larger dimensions of the blade); and    -   the new cross-compound turbo-unit group of twice the power as        compared to the two turbo-units according to the known art will        have construction and management costs of that are considerable        lower, owing to the smaller number of components (the number of        the main valves and of the regulation and protection systems is        reduced to one half).

Finally, the system and the method according to the invention enablecontrol of the system to be carried out in a simplified way, controllingwith sufficient precision the speed of the two shafts via the regulationvalve set in series along the branch. In particular, it is possible tolimit the overspeed of the corresponding line of shafts in the case oftotal loss of the electrical load or, in any case, to satisfy inoperation any possible requirements of regulation of the flowratetowards said section. Thus the known problem linked to the considerableamount of steam contained in the line having non-controllable flowrate,which might cause situations that are dangerous for the mechanicalintegrity of the line of shafts itself, is overcome.

Finally, it is possible to apply the solution proposed on systems withthe two lines of shafts set even at a considerable relative distanceapart from one another, something which is not possible with thecross-compound configurations of the known art.

BRIEF DESCRIPTION OF THE DRAWING

Further purposes and advantages of the invention will emerge clearlyfrom the following description of an embodiment thereof, provided purelyby way of non-limiting example and with reference to the FIGURE of theannexed plate of drawings, which is a schematic illustration of a systemfor the production of energy in cross-compound configuration on twoshafts.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the aforesaid single FIGURE, designated as a whole by1 is a system for the production of electrical energy by means of atleast one pair of steam turbines 2, 3 of similar size, operating incross-compound configuration. In particular, the system 1 comprises afirst steam turbine 2, which is coupled on a first shaft 4 to a firstelectric generator 6, and a second steam turbine 3, which is coupled ona second shaft 5 to a second electric generator 7. The generators 6, 7are then connected in a way that is known and in any case obvious topersons skilled in the art, to an electrical network 9.

The steam turbine 2 includes just one high-pressure section 10 and onefirst low-pressure section 12, whilst the steam turbine 3 includes justone medium-pressure section 14 and one second low-pressure section 16.

The high-pressure section 10 is exclusively connected, according to anaspect of the invention, to the medium-pressure section 14, located onthe other shaft (the section 10 is on the shaft 4, whilst the section 14is on the shaft 5), upstream thereof, whilst the medium-pressure section14 is connected in parallel to both the first low-pressure section 12and the second low-pressure section 16, immediately upstream thereof.

Hereinafter, the terms “upstream” and “downstream” refer to thedirection of flow of the operating fluid (steam) in the system 1,indicated schematically by the arrows in the FIGURE.

In particular, the medium-pressure section 14 is connected to thelow-pressure sections 12 and 16 by means of a piping 18 of a relativelyshort length, which connects the medium-pressure section 14 directly tothe low-pressure section 16, which is located on the same shaft 5, andby means of a branch 20 of the piping 18, of a relatively long length,which connects the piping 18 to the low-pressure section 12, which islocated on the shaft 4 provided with the high-pressure section 10.

A regulation valve 21 intercepts in series the branch 20, as near aspossible to the piping 18.

The system 1 further comprises a single supercritical boiler 22, whichsupplies in cascaded fashion the high-pressure section 10 and, then, themedium-pressure section 14. The boiler 22 for the production of steam inturn comprises at least one superheater element 23 and at least onere-superheater element 24, constituted by nests of pipes arranged withina single casing to form the boiler 22. A first branch 30 of hydrauliccircuit connects the superheater 23 to the high-pressure section 10 ofthe steam turbine 2. A second branch 32 of hydraulic circuit connectsthe high-pressure section 10 to the medium-pressure section 14 of thesteam turbine 3. The re-superheater 24 is hydraulically inserted inseries along said branch 32.

The low-pressure sections 12 and 16 discharge the exhaust steam towardscondensers 40 and 41, respectively. The valve 21 can be, as may be seen,an on-off valve, or else a proportional valve; in any case, it iscontrolled by an electronic control unit 45 for controlling rotation ofthe shafts 4 and 5. The electronic control unit 45, indicatedschematically as a block in the FIGURE, is in actual fact a complexsystem (which is on the other hand known or in any case of a typeobvious for a person skilled in the art and consequently is notdescribed in detail herein) for regulation/protection that controls alsothe new main valves of the HP and MP sections (indicated in the FIGUREas dashed and identified by the reference numbers 104 and 105,respectively) and not only rotation of the shafts; it moreover controlsalso (and not only) the pressure of the steam.

According to the method of the invention, the system 1 is obtained byoperative reconversion of a pair of pre-existing steam turbo-units ofsimilar size, such as for example the two units constituted respectivelyby the turbine 2, with the shaft 4 and the generator 6, and by theturbine 3 with the shaft 5 and the generator 7, which are coupled to oneanother in an innovative configuration of a cross-compound type. In thiscase, both of the pre-existing turbines 2 and 3 comprise, instead of thesingle sections 10 and 14, normal HP-MP units; i.e., each comprises ahigh-pressure section 50 and a medium-pressure section 51, and each issupplied originally by an independent subcritical boiler provided with asuperheater 52 and a re-superheater 53.

The method of the invention then comprises: a step of decommissioningand removal of the high-pressure and medium-pressure sections 50, 51 ofboth of the steam turbines 2 and 3; and a step of replacement of theremoved sections 50, 51 with a new high-pressure section 10 on the firstshaft 4 and with a new medium-pressure section 14 on the second shaft 5.

According to the invention, however, said new high-pressure section 10and new medium-pressure section 14 are made in such a way as to occupyat least (a fair) portion of the space left free, on each shaft 4, 5, byboth the high-pressure and medium-pressure sections 50, 51 removed fromthat shaft, leaving the respective pre-existing foundations 70 of eachshaft 4, 5 unaltered.

The method of the invention further envisages a step of setting inhydraulic connection in cascaded fashion the new high-pressure section10 on the first shaft 4 and the new medium-pressure section 14 on thesecond shaft 5 with a single supercritical boiler 22, so that the latterwill be able to supply one and the same flow of steam V (indicated bythe arrow), in series, to the new high-pressure section 10 and, then, tothe new medium-pressure section 14.

The method according to the invention further envisages a step ofsetting in hydraulic connection in parallel exclusively the newmedium-pressure section 14 with the respective pre-existing low-pressuresections 12, 16 of both of the turbines 2 and 3, set respectively on thefirst shaft 4 and on the second shaft 5, by means of the single piping18 set downstream of the new medium-pressure section 14 to collectsubstantially the entire flow of steam V that traverses the latter andsupply it in parallel, directly, to the pre-existing low-pressuresection 16 of the second shaft 5 and, via the branch 20 intercepted bythe regulation valve 21, to the pre-existing low-pressure section 12 ofthe first shaft 4.

Obviously, the method of the invention envisages also a step of removalof the respective subcritical boilers for actuation of the first andsecond turbines 2, 3 (hence with removal of the superheaters andre-superheaters 52, 53) and installation of the single supercriticalboiler 22 for actuation of both of the turbines 2, 3.

The single supercritical boiler 22 is provided with at least thesuperheater 23 and the re-superheater 24, cascaded to one another, theformer hydraulically connected in series to the new high-pressuresection 10 of the first shaft 5 upstream thereof, and the latterhydraulically connected in series to the new medium-pressure section 14of the second shaft 5, immediately upstream thereof and immediatelydownstream of the new high-pressure section 10.

The regulation valve 21 is set on the branch 20 as near as possible tothe piping 18 that connects the branch 20 and the pre-existinglow-pressure section 16 of the second shaft 5 to the new medium-pressuresection 14 of the second shaft 5 itself. The regulation valve 21 can be,as has already been said, an on-off valve, or else a proportional valvecontrolled by the electronic control unit 45, programmed to perform, ina way that is known to a person skilled in the art, functions of controland protection for both the first shaft 4 and second shaft 5, as hasbeen more fully specified previously.

From the foregoing description, it is evident how the solution ofreplacing on pre-existing turbines the traditional units ofhigh-pressure and medium-pressure sections with single high-pressure ormedium-pressure sections (one on one shaft and the other on the othershaft) of substantially the same overall dimensions, enables readyadaptation of the regulation of the cross-compound turbo-unit and of theelectrical part of the generators, with similar powers on the twogenerators. It will simply be necessary to have:

1—a mechanical and vibrational re-design of the two lines of shafts,constituted each by a set of new parts and existing parts to enabletheir operation in normal and emergency conditions;

2—a new design of the pipes that connect the single MP section to thetwo existing LP sections; in particular, on the line downstream of thebranching towards the LP section most distant from the new MP section,there is envisaged insertion of the valve 21, for example with automaticcontrol, which:

A) enables limitation of the overspeed of the corresponding line ofshafts in the case of total loss of the electrical load, or in any casemakes it possible to satisfy in operation any possible requirements ofregulation of the flowrate towards said section;

B) enables, also for the reasons explained above in point A),application of the proposed solution on systems with the two lines ofshafts at a considerable relative distance apart;

3—maintenance of existing foundations, generators, LP sections andcondensers;

4—a new and single control and protection system for both of the shaftsof the turbine, such as to enable, on the basis of the new re-designingmentioned in the previous points, with particular reference to theprotections corresponding to the overspeed;

5—a new design of the instrumentation and of the monitoring of the twoexisting units to enable regular operation thereof as a single unit; and

6—a re-design of the electrical part to enable operation of the twogenerators (envisaged for independent operation) as a single unit(generation in parallel).

1. A method for obtaining operative reconversion of a pair ofpre-existing steam turbo-units, in which at least one pair of said steamturbo-units, a first of which comprising a first steam turbine (2) and afirst electric generator (6) connected on a first common shaft (4) and asecond of which comprising a second steam turbine (3) and a secondelectric generator (7) connected on a second common shaft (5), arecoupled to one another in a configuration of a cross-compound type; saidmethod being characterized in that it comprises at least: a step ofdecommissioning and removal of the high-pressure and medium-pressuresections of both of said steam turbines; a step of replacement of theremoved sections with a new high-pressure section (10) on the firstshaft and a new medium-pressure section (14) on the second shaft, saidnew high-pressure and medium-pressure sections being made in such a wayas to occupy at least part of the space left free, on each shaft, byboth the high-pressure section and the medium-pressure section removedfrom that shaft, leaving the pre-existing foundations unaltered; and astep of setting in hydraulic connection in cascaded fashion the newhigh-pressure section on the first shaft and the new medium-pressuresection on the second shaft to a single supercritical boiler (22), insuch a way that the latter will be able to supply one and the same flowof steam, in series, to the new high-pressure section and, then, to thenew medium-pressure section.
 2. The method according to claim 1,characterized in that it further comprises a step of setting inhydraulic connection in parallel only the new medium-pressure sectionwith respective pre-existing low-pressure sections (12, 16) of both saidfirst turbine and said second turbine, set respectively on the firstshaft and on the second shaft, by means of a single pipe (18) setdownstream of the new medium-pressure section to collect substantiallyall the flow of steam (V) that traverses the latter and supply it inparallel, directly, to the pre-existing low-pressure section (16) of thesecond shaft and, via a branch (20) intercepted by a regulation valve(21), to the pre-existing low-pressure section of the first shaft. 3.The method according to claim 1, characterized in that it comprises astep of removal of respective subcritical boilers, of actuation of saidfirst and second turbines, and of installation of a single saidsupercritical boiler for actuation of both of the turbines.
 4. Themethod according to claim 3, characterized in that said singlesupercritical boiler is provided with at least one superheater (23) andat least one re-superheater (24), cascaded to one another; the formerbeing hydraulically connected in series to said new high-pressuresection of the first shaft upstream thereof and the latter beinghydraulically connected in series to said new medium-pressure section ofthe second shaft, immediately upstream thereof and immediatelydownstream of the new high-pressure section.
 5. The method according toclaim 2, characterized in that said regulation valve (21) is set on thebranch as close as possible to the pipe that connects the branch and thepre-existing low-pressure section of the second shaft to the newmedium-pressure section of the second shaft.
 6. The method according toclaim 5, characterized in that said regulation valve is an on-off valve.7. The method according to claim 5, characterized in that saidregulation valve is a proportional valve controlled by an electroniccontrol and protection unit (45) for both said first shaft and saidsecond shaft.
 8. A system (1) for the production of energy by means ofat least one pair of steam turbines (2, 3) operating in cross-compoundconfiguration, in which a first steam turbine (2) is coupled on a firstshaft (4) to a first electric generator (6), and a second steam turbine(3) is coupled on a second shaft (5) to a second electric generator (7);and in which the first steam turbine includes just a high-pressuresection and a first low-pressure section, whilst the second steamturbine includes just a medium-pressure section and a secondlow-pressure section; wherein said high-pressure section is connectedexclusively to the medium-pressure section, upstream thereof, whilst themedium-pressure section is connected in parallel to said first andsecond low-pressure sections, immediately upstream thereof, by means ofa pipe (18) of a relatively short length that connects themedium-pressure section (14) of the second steam turbine to the secondlow-pressure section, which is located on the same shaft, and, by meansof a branch (20) of said pipe, of a relatively long length, thatconnects said medium-pressure section (14) with the first low-pressuresection, which is located on the shaft of said high-pressure section;wherein said system further comprises a regulation valve (21) thatintercepts in series said branch, said regulation valve being placedalong said branch as near as possible to said pipe; and in that saidlow-pressure sections discharge the exhaust steam towards a differentcondenser respectively.
 9. The system according to claim 8,characterized in that it further comprises a single supercritical boiler(22), which supplies in cascaded fashion, said high-pressure sectionand, then, said medium-pressure section.